Vehicles may be fitted with evaporative emission control systems to reduce the release of fuel vapors to the atmosphere. For example, vaporized hydrocarbons (HCs) from a fuel tank may be stored in a fuel vapor canister packed with an adsorbent which adsorbs and stores the fuel vapors. At a later time, when the engine is in operation, the evaporative emission control system allows the fuel vapors to be purged into the engine intake manifold from the fuel vapor canister to be consumed during combustion.
In one example described in U.S. Pat. No. 5,398,660, a fuel vapor canister includes a plurality of purge valves and a plurality of air inlet valves. During operation of the engine, all of the purge valves and the air inlet valves may be opened to supply a negative pressure from an engine air induction passage to within the canister. As a result of the supply of the vacuum, fuel vapor is purged to the intake manifold of the engine from the fuel vapor canister.
However, the inventors herein have recognized issues with the above approach. For example, in engine applications that operate with low vacuum air induction, by opening all air inlet and purge valves of the fuel vapor canister at the same time, a small amount of vacuum may be created in the fuel vapor canister. Accordingly, the amount of time it takes for the fuel vapor canister to be purged may be substantial. More particularly, in hybrid electric vehicle (HEV) applications, the engine run time may be shorter than the amount of time it takes to purge the fuel vapor canister with low vacuum.
Thus, in one example, the above issues may be addressed by a method for operating a fuel system comprising: sequentially purging fuel vapors from each of a plurality of regions of a canister. Specifically, purging a region of the canister may include opening an air inlet valve associated with that region and maintaining air inlet valves associated with each other region of the canister closed in order to direct fuel vapors to at least one purge outlet of the canister.
In one example, a region of the canister may be purged until a fuel fraction of combustion gases exhausted from the cylinders is less than a set point. Once a region has been purged to the set point, the associated air inlet valve may be closed and an air inlet valve associated with a next region in the sequence may be opened while maintaining each of the other air inlet valves closed to purge that region.
By opening one air inlet valve at a time, air flow through the region of the canister associated with that air inlet valve may be increased to more quickly purge fuel vapors from that region to meet the set point. In this way, the amount of time to purge the canister may be reduced relative to the approach where all valves are opened at the same time. Moreover, the increased air flow may purge the region more thoroughly relative to a purge approach with lower air flow. In other words, the increased air flow may increase the likelihood of attaining zero bleed emissions from the canister.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.